1. Field of the Invention
The present invention relates to a light wavelength converting system which includes a light wavelength converting element, a light wavelength converting module, and a wavelength stabilized laser.
A first aspect of the present invention relates to a light wavelength converting element, and more specifically to an optical waveguide-type light wavelength converting element in which an optical waveguide is formed on a substrate having a nonlinear optical effect.
Further, the present invention relates to a light wavelength converting module in which such a light wavelength converting element as described above and an external resonator-type semiconductor laser which makes a laser beam as a fundamental wave enter the light wavelength converting element are connected to each other.
A second aspect of the present invention relates to a wavelength stabilized laser in which a semiconductor laser beam is passed through a band-pass filter to thereby stabilize an oscillating wavelength thereof.
A third aspect of the present invention relates to a light wavelength converting module which converts a wavelength of a fundamental wave which is emitted from a semiconductor laser by using the light wavelength converting element. More specifically, the third aspect of the present invention relates to a light wavelength converting unit which is provided with a semiconductor laser having an external resonator and with a light wavelength converting element which converts the wavelength of a fundamental wave which is emitted from this semiconductor laser into a second harmonic or the like, and to a light wavelength converting module having this light wavelength converting unit.
2. Description of the Related Art
Regarding the first aspect of the present invention, conventionally, for example, as disclosed in Japanese Patent Application Laid-Open (JP-A) No. 10-254001, there has been known a light wavelength converting element in which an optical waveguide which extends unilaterally is formed on a ferroelectric crystal substrate having a nonlinear optical effect, in which domain inverting portions in which orientations of spontaneous electrodes of the substrate are inverted are periodically formed on the optical waveguide, and in which a fundamental wave, which, within the optical waveguide, guides waves in a direction in which the domain inverting portions are arrayed, is converted into a second harmonic or the like.
This publication (JP-A No. 10-254001) also discloses a light wavelength converting module which connects the aforementioned light wavelength converting element and a semiconductor laser which makes a laser beam as a fundamental wave enter this light wavelength converting element. Further, this publication also discloses a technique in which the aforementioned semiconductor laser and an external resonator having a wavelength selecting element such as a narrow band-pass filter are combined. Due to the operation of the external resonator, a wavelength which is transmitted from the semiconductor laser is locked at a predetermined wavelength.
JP-A No. 6-160930 discloses a light wavelength converting element in which an optical waveguide is formed on a substrate having a nonlinear optical effect to thereby convert a wavelength of a fundamental wave which enters from one end surface side (proximal end surface) of the optical guide and emit a converted wavelength wave from the other end surface (distal end surface) thereof. The proximal end surface and/or the distal end surface of the optical waveguide are formed so as to be inclined in a vertical direction (namely, with respect to a plane orthogonal to a direction in which the optical waveguide extends, within a plane orthogonal to the surface of the substrate, which surface includes this direction). In this structure, the fundamental wave which structures the optical waveguide and which enters the aforementioned proximal end surface and/or the distal end surface is reflected outwardly from these end surfaces at an angle with respect to the direction in which the optical waveguide extends. Therefore, this fundamental wave is prevented from re-entering the optical waveguide and from becoming a fed-back light which enters the semiconductor laser.
In the light wavelength converting module which is structured as described above, generally, a reflection preventing coating (AR (acid resisting) coating) is applied to one end surface and the other end surface including the optical waveguide end surface of the light wavelength converting element. However, such an AR coating does not exhibit a perfect reflectance of 0%. In a case of an SiO2 single-layered coating formed by using a vacuum evaporation device with an ordinary spectral reflection monitor, the reflectance is 0.05% to 0.1%.
Accordingly, the fundamental wavelength is reflected slightly at an emitting end of the light wavelength converting element and then returns to the semiconductor laser, thus adversely affecting oscillation of the semiconductor laser. A more detailed description of the problem due to the fed-back light to the semiconductor laser will be given hereinafter with reference to a case in which a second harmonic is generated by using an external resonator-type semiconductor laser as a fundamental wave light source.
When the driving current of the semiconductor laser is increased, the temperature of the semiconductor laser varies, and the wavelength thereof changes, and the oscillating wavelength thereof changes periodically in the vicinity of a central wavelength selected by a wavelength selecting element. An example of such a change as described above is shown in FIG. 8. This is an example of a case in which the oscillating wavelength and the wavelength of the semiconductor laser are about 950.0 nm and 1 mm, respectively, and the transmitting central wavelength and the transmitting width of the narrow band-pass filter which is the wavelength selecting element are 950.0 nm and 0.5 nm, respectively.
As described above, the light which is reflected and returns to the semiconductor laser and the light which proceeds to the emitting end of the optical-waveguide interfere to each other. However, since an optical path length inside the light wavelength converting element is fixed, the amount (intensity) of the light which returns to the semiconductor laser varies periodically in accordance with the change of the wavelength. When the amount of light which returns to the semiconductor is great, a vertical mode hop occurs or the oscillation strength becomes unstable. For this reason, for example, as shown in FIG. 9, the current vs. light output characteristic (referred to as IL characteristic hereinafter) of the second harmonic swells or the amount of light varies unstably (noise is generated). In a semiconductor laser having such a characteristic as that illustrated in FIG. 9, a problem arises in that, when an APC (automatic power control) or an output stabilizing control is applied to the semiconductor laser, the variation in the amount of light over time becomes large, namely, the noise increases. When such a light wavelength converting module is used as a recording light source for a laser scanner, for example, the problem arises that more noise is generated in the recorded image.
In particular, since the semiconductor laser whose oscillating wavelength has been locked by the above-described external resonator has high monochromaticity, it is easy for the above-described interference to occur, and the problem due to the fed-back light is likely to be caused.
The light wavelength converting element which is disclosed in the above-described JP-A No. 6-106930 has been provided in order to overcome such problems due to fed-back light. However, since the other end surface of the optical waveguide is formed so as to be inclined in a vertical direction, in order to avoid the effect of the fed-back light, this inclination should be made relatively large. Therefore, in this light wavelength converting element, there arises the problem that the converted wavelength wave emitted from the other end surface of the optical waveguide is reflected largely, thus making it difficult to align optical axis of the laser beam with other optical elements.
In a case in which the proximal end surface is inclined as described above, a problem arises in that the light emitted from the semiconductor laser does not efficiently enter the light wavelength converting element.
With regard to the second aspect of the present invention, conventionally, as disclosed in JP-A No. 10-186427, there has been known a wavelength stabilized laser in which the laser beam emitted from the semiconductor laser is passed through the band-pass filter, and light is fed back to the semiconductor laser to thereby stabilize the oscillating wavelength of the semiconductor laser.
The wavelength stabilized laser is fundamentally structured by a semiconductor laser, a collimator lens which makes parallel a laser beam which is emitted in a state of a divergent light from this semiconductor laser, a condenser lens which converges the laser beam which has been made parallel by the collimator lens, a means which returns the converged laser beam to the semiconductor laser, and a band-pass filter which is disposed between the collimator lens and the condenser lens and through which only light of predetermined wavelengths is passed.
In the wavelength stabilized laser, the laser beam which has been selected by the band-pass filter is returned to the semiconductor laser so that the oscillating wavelength of the semiconductor laser is stabilized.
However, in the conventional wavelength stabilized laser having the aforementioned structure, there is a problem in that the linearity of the IL characteristic is unsatisfactory. The present inventors studied the reasons for this and arrived upon the results described below.
In the wavelength stabilized laser having the above-described structure, when an optical member such as a mirror or the like is inserted into the optical path of the laser beam which is emitted from the semiconductor laser, laser beams which have been reflected from the end surfaces of the mirror return to the semiconductor laser. Therefore, from this semiconductor laser, rays of light having different optical path lengths merge together and emitted. In this way, the rays of light having different optical paths interfere with each other. When a driving current which is applied to the semiconductor laser is varied, heat is generated by the semiconductor laser such that the refractive index and the length change, and the oscillating wavelength changes as well. Thus, when the oscillating wavelength changes, since the above-described state in which the rays of light interfere with each other also changes, the linearity of the IL characteristic may deteriorate.
With regard to the third aspect of the present invention, conventionally, an excitation solid-state laser, which excites a solid-state laser crystal by light which is emitted from a laser diode, has been used as light sources of a blue color laser (473 nm) or a green color laser (532 nm). These excitation solid-state lasers, as shown in FIG. 36, include a laser diode 110 in a transverse multi-mode which emits a laser beam 100 as excited light, a condenser lens 112 which converges the laser beam, a solid-state laser crystal 114, a mirror 116 which is disposed ahead of the solid-state laser crystal and which forms an incident side end surface of the solid-state laser crystal 114 and an internal resonator, an SHG crystal 118 for generating a second harmonic, which is disposed between the solid-state laser crystal 114 and the mirror 116 and which has a periodic domain inversion structure, a Brewster's plate 119 which is disposed between the solid-state laser crystal 114 and the SHG crystal 118, and an etalon 120 which is disposed between the SHG crystal 118 and the mirror 116.
Such an excitation solid-state laser uses laser crystals and the like such as a YAG crystal and a YVO4 crystal which are solid-state laser media and on which a rare earth element such as neodymium (Nd) has been doped. However, not only the laser crystals limit an oscillating wavelength to a predetermined wavelength, but also the laser crystal itself exhibits low response frequencies, for example, 100 kHz with the YAG crystal and 2 MHz with the YVO4 crystal. Accordingly, there has been the drawback in that rapid modulation cannot be performed with an excitation solid-state laser. Further, in order to structure the internal resonator, reflection films or AR films, which correspond to an incident wavelength, a resonating wavelength, and an emitting wavelength, respectively, must be provided at the solid-state laser crystalline or the member which forms the resonator, and the manufacturing of the internal resonator becomes complicated. Moreover, due to the use of the laser diode in the transverse multi-mode, a transverse mode hop is caused which causes noise.
Further, in such an excitation solid-state laser, a resonator structure such as a Fabry-Perot type resonator or a ring resonator is adopted. However, a problem arises in that, due to changes in the humidity or atmospheric pressure in the environment in which the laser is used, the length of the resonator changes, and an oscillating wavelength thereby varies. In particular, etalon is apt to be influenced by the environment in which the laser is used. For this reason, JP-A No. 9-266338 discloses an excitation solid-state laser in which a solid-state laser crystal, a laser diode, and all of the optical element forming a resonator are stored or hermetically sealed in a container whose interior is maintained in an airtight state. Changes in the length of the resonator due to changes in humidity or atmospheric pressure are eliminated, thus preventing variations in the output of the solid-state laser and the oscillating wavelength. However, in this excitation solid-state laser, there has been a problem that since all of the main structural components must be hermetically sealed, the device becomes bulky and the manufacturing cost thereof becomes high. Further, there has been a problem in that, when the number of components to be hermetically sealed is large, a mirror or the like deteriorates over time due to gasses which are generated from the respective components so that the output from the solid-state laser thereby deteriorates.